The present invention relates generally to differential amplififers, and in particular to a differential shunt feedback amplifier with high common-mode rejection.
Differential amplifiers are well known in the art and are used in many applications in which it is desired to amplify only the differential signal component (the instantaneous difference in signal voltages between a pair of inputs) while rejecting the common-mode component of an input signal. In such amplifiers, one measure of accuracy is the degree to which the common-mode signal component can be cancelled or rejected, and this is known as common-mode rejection (CMR).
One common technique to provide a high common-mode rejection ratio for differential amplifiers comprising an emitter-coupled pair of transistors is to return the common emitters to a reference potential via a very high impedance, such as that exhibited by a constant current source. This technique is known in the art as "long-tailing", and allows the emitter voltage to vary with the common-mode signal without developing a common-mode signal current in the transistors. Thus the collector voltages remain constant with respect to the common-mode signal, effectively rejecting the common-mode signal. However, the utility of this type of circuit is limited because the gain is determined by the characteristics of the transistors themselves and is therefore uncontrollable and unpredictable, and the output impedance is either very high or determined by collector load impedances.
Another common technique to provide a high common-mode rejection ratio for differential amplifiers in general is to tap the common-mode signal from the center of a resistive voltage divider connected across the two inputs to the differential amplifier, apply the common-mode signal to an error amplifier, and then combine in some manner the error signal thus developed with the differential amplifier signal to cancel the common-mode component thereof. However, this type of corrected differential amplifier is complex and generally has a comparatively high power consumption.